Actions will lead to a completely new class of productivity workflows on iOS because of the integrated and secure extension model Apple has designed. Imagine utilities like an Evernote web clipper for Safari, an Instapaper extension, or something as trivial as printing a webpage to PDF: until today, the limitations of iOS forced developers to create standalone apps unable to talk to each other. With Actions, app features become visual extensions that share resources with their containing app (i.e. an Evernote clipper would authenticate with your account from the Evernote app) and that let you do more without sacrificing the simplicity of iOS.

Extensions is probably my big thing from WWDC. I'd compare it to when Apple added multitasking support without killing the battery life. It seems Apple's extension implementation opens a world of possibilities for apps while not significantly compromising security and system resources. I'm very interested to see how my favorite apps implement new features and what new apps may sprout up to leverage extensions.